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You can also do them with the formula
(y-y1)/(y2-y1) = (x-x1)/(x2-x1)
So:
(y+1)/(5/2+1) = (x+1/2)/(3+1/2)
y+1 = x+1/2
y = x - 1/2
and
(y+3) /(9+3) = (x-6)/(-1-6)
(y+3) / 12 = (x-6) / -7
-7y -21 = 12 x -72
y = -12x/7 +51/7